Most spectrophotometers allow to vary the widths of the entrance slit and of the exit slit. The slit width is increased when the amount of light entering the spectrophotometer is to be increased, for example in order to improve the signal-to-noise ratio. The increase of the slit width, however, has the consequence that the spectral resolution of the spectrophotometer is deteriorated. On the other hand, the resolution of a spectrophotometer can be improved, at the expense of the signal-to-noise ratio, by reducing the slit width.
There are various prior art solutions for achieving variability of the slit width. The simplest approach is to provide a replaceable slit which an operator can replace manually by a slit of a different width. This approach is of course cumbersome and inflexible so that slits with adjustable widths have been developed. A disadvantage of the existing adjustable slits, however, is that they are complex and costly. An adjustable slit is known, for example, from U.S. Pat. No. 3,394,977. According to this prior art, the entrance slit and the exit slit each have a fixed and a movable slit edge, wherein the movable edge is moved by means of a resiliently flexible arm. The slit adjustment mechanism is designed such that the widths of the entrance slit and of the exit slit are varied simultaneously. The known mechanism comprises a very large number of individual parts and is thus very complex, resulting in an increased susceptibility to trouble and high cost. Another disadvantage is that the center of the slit varies when the slit width is varied because one of the slit edges remains fixed. Furthermore, the adjustment of the slit width has to be performed manually by an operator.
Another adjustable slit system is known from U.S. Pat. No. 3,508,813. This slit system comprises a metalized quartz disc in which a plurality of slits of various widths are concentrically arranged. Two opposite slits have the same width, respectively and thus form a slit pair which serves as entrance slit and exit slit of a monochromator. Different slit widths can be selected in discrete steps by rotating the disc by a defined angle, respectively. The center of the slit is substantially preserved. However, the reproducibility in the adjustment of the slit centers may not be sufficient for very great demands on the spectroscopic accuracy.
From U.S. Pat. No. 3,457,004, an adjustable slit for a spectroscopic apparatus is known wherein a lever mechanism is provided for opening the two edges of the slit symmetrically from the center of the slit. This construction is mechanically complex, leading to high cost and increased probability of failure.